The present invention generally relates to recovery operations at sea and, more particularly, to a system for limiting the intensity of snap loads introduced in a tether or cable used in the recovery operations at sea.
A surface support craft mounting a crane is commonly used in recovery at sea of objects, such as inoperative vehicles and submersible systems. During surface handling phases of recovery operations, snap loads can be introduced into the crane's cable caused by rolling and heaving of the surface support craft. Snap loads occur when there is a slack in the cable which produces a separation velocity between the object and the crane tip. These snap loads can introduce abnormally high tensile loads into the cable which, if severe enough, can part the cable resulting in loss of the object and/or damage to the crane or injury to the personnel.
Several approaches have been used in the past to isolate the cable from shock loads; however, all have disadvantages. One approach is to strengthen the cable sufficiently to withstand any snap loads that may occur. This approach may be reasonable in situations where the objects are small and light, however, not for larger, heavier objects. To increase cable strength requires increasing the cable diameter to the point where either the cable handling equipment or the cable drag becomes unacceptable. Also, though this approach insures against loss of the object, damage can still occur to the crane, other surface handling equipment, or the object since the snap load is still transmitted through the cable directly to these items.
Another approach is to provide a complex motion compensation system in the surface handling crane. This system compensates for the crane tip motion caused by movement of the surface support craft. However, the compensation system increases the overall size of the crane, limiting its utilization on most surface craft.
Still another approach is to utilize a pneumatic spring on the object to isolate the object and the cable. The primary disadvantage with this approach is that its effectiveness is dependent on the depth of the object. At depth, the increased ambient pressure compresses the pneumatic spring and eliminates its effectiveness in isolating the object from the shock of snap loads. A reliable system must be equally effective at different depths since snap loads can occur anywhere during recovery of the object.
In view of the above-described disadvantages of prior approaches, there is still a need for a reliable system for minimizing snap loads on the crane lift cable. The system must be effective independent of package size and depth.